1. Field of the Invention
The present invention relates to a pulse signal generating device of a direct-current (DC) electric motor used for office automation appliances.
2. Description of the Prior Art:
In recent years, a thinner and less expensive floppy disk drive (FDD) has been realized. To be adaptable for such a floppy disk drive, a DC motor pulse signal generating device is also required to be thin and inexpensive while the efficiency thereof is enhanced. To achieve this, it is essential for the DC motor pulse signal generating device to generate pulses with a stabilized waveform and minimize leakage of magnetic flux.
A conventional DC motor pulse signal generating device will be described with reference to FIGS. 5 and 6. FIG. 5 shows a vertical sectional view of a half part of a conventional DC motor pulse signal generating device which comprises a rotor including a shaft 19, a housing 21 fitted in the shaft 19, and a rotor frame 23 fixed to the housing 21 with caulking. A Teflon sheet 20 is attached to the housing 21. A hub magnet 22 and a frequency generating (FG) magnet 25 are formed by integral molding with the rotor frame 23. A rotor magnet 24 is attached to the inner surface of the rotor frame 23. A pulse generating (PG) magnet 29 is fitted in a groove formed on the circumferential side face of the FG magnet 25. The top surface of the PG magnet 29 is covered with a shield plate 26 so that leakage of magnetic flux from the PG magnet 29 is minimized.
The DC motor pulse signal generating device also comprises a printed board 30 to which a core 32 and a bearing 34 are fixed with a pin 33. A coil 31 is wound around the core 32. A magnetic sensor 27 for generating magnetic index pulses is also fixed to the printed board 30 through a support of a holder 37 at a position appropriately spaced apart from the circumferential end face of the FG magnet 25. A terminal of the magnetic sensor 27 is fixed to the printed board 30 with a solder 28.
The operation of the conventional DC motor pulse signal generating device with the above-described structure will be described.
FIG. 6 is an enlarged view of a main portion of FIG. 5. The magnetic sensor 27 detects a magnetic flux from the PG magnet 29 fitted in the groove of the rotating FG magnet 25 once every rotation thereof. The waveform of the detected flux is shaped through an amplifying circuit and a Schmitt trigger circuit known in the art, so that a pulse signal is generated every rotation.
In the conventional pulse signal generating device, the height T of the FG magnet 25 from the surface of the printed board 30 is restricted to a certain level so that the FG magnet 25 does not hit the magnetic head of an FDD. As a result, the positions of the magnetic center of the PG magnet 29 shown by a reference numeral 35 and the center of the magnetic sensor 27 shown by a reference numeral 36 are not on the same level, but the former is lower than the latter. Accordingly, the strongest magnetic flux generated from the magnet center 35 of the PG magnet 29 is not effectively detected by the center 36 of the magnetic sensor 27. FIGS. 3 and 4 show enlarged views of the PG magnet and the magnetic sensor in perspective, respectively, for reference.
To make the magnetic sensor operative under the above-described condition, it is necessary to use a magnet having a high magnetic energy product, such as samarium cobalt and neodymium iron with a high residual flux density. The use of such magnets results in a cost increase and also leakage of magnetic flux from the PG magnet 29 made of these magnets is increased.
Furthermore, in the conventional pulse signal generating device, the holder 37 is used to fix the magnetic sensor 27 to the printed board 30. This requires additional cost. Moreover, since the magnetic sensor 27 is not provided with a positioning mechanism, it tends to incline itself, and also an air gap G between the magnetic sensor 27 and the PG magnet 29 tends to vary both within a device and between devices. As a result, pulses generated from the magnetic sensor 27 can not be stabilized.
The objective of the present invention is to provide an inexpensive DC motor pulse signal generating device capable of generating stable pulses with reduced leakage of flux.